Fuel cells generate electrical power that can be used in a variety of applications. Fuel cells constructed with proton exchange membranes (PEM fuel cells) may eventually replace the internal combustion engine in motor vehicles. PEM fuel cells have an ion exchange membrane, partially comprised of a solid electrolyte, affixed between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen is supplied to the anode and air is supplied to the cathode. An electrochemical reaction between the hydrogen and the oxygen in the air produces an electrical current, with water and heat as reaction products. This water is removed from the cathode.
The ion exchange membrane commonly used in PEM fuel cells is partially comprised of a sulphonated chemical compound that binds water in the membrane in order to ensure sufficient proton conductivity. At ambient temperatures below zero degrees Celsius, water contained in the membrane can freeze. When such freezing occurs, the electrical resistance of the membrane can increase by two to three orders of magnitude.
Conventional fuel cells can only produce current at temperatures above a defined starting temperature, which is currently approximately 5 degrees Celsius. In the event of a cold start (i.e. temperatures at or below approximately 5 degrees Celsius), a fuel cell must first be heated to a temperature above the starting temperature. Because of the considerable mass of fuel cell components, the required increase in temperature can only be achieved if a large amount of thermal energy is realized and such thermal energy is then transferred to fuel cells. In current fuel cell systems, it is not possible to start a frozen fuel cell within times similar to those achieved for internal combustion engines. Further, due to the expansion of water in a solid phase (ice), freezing will inevitably cause damage to the delicate porous structure of a fuel cell and, consequently, a degradation of its performance.
Considerable effort has been directed toward accelerating the rate at which a PEM fuel cell system can be heated to above-freezing temperatures. For instance, the introduction of an H2/air mixture into a fuel cell stack can be used to initiate an exothermal chemical reaction. According to U.S. Pat. No. 6,127,056, during start up, a fuel cell is warmed to operating temperature by introducing a small amount of the hydrogen into an air flow at an air inlet of the fuel cell where the hydrogen and the oxygen react at a catalyst surface to produce heat. The drawback of this approach is that the warmed substance is remote relative to the cathode where the typical electrochemical reaction occurs under the general operation of the fuel cell. In U.S. Pat. No. 6,103,410, an H2/air mixture is delivered into oxidant channels and reacts on a catalyst disposed in hydrophobic regions of the cathode. U.S. Pat. No. 6,358,638 teaches a method in which a membrane electrode assembly (MEA) is locally heated from below freezing to a suitable operating temperature by the exothermal chemical reaction between H2 and O2 on the anode and/or cathode catalysts. The hydrogen is introduced into an O2-rich cathode feed stream and/or O2 is introduced into a H2-rich anode feed stream. One considerable disadvantage of the approach in the '410 and '638 patents is the fact that a heated zone is mainly restricted to an electrode area in the proximity to a fuel cell inlet. Overheating eventually causes damage to fuel cell components. In all the above-mentioned systems, auxiliary apparatus is needed to provide gas mixture preparation.
In U.S. Patent Application Publication No. 2004/0013915 A1, a PEM fuel cell is heated from a temperature below the freezing point (0 degrees Celsius) by supplying hydrogen to an anode so as to form water by combining with oxygen generated at the anode by the electrolysis of the frozen water. Further, the oxygen is supplied to the cathode so as to form water by combining with the hydrogen generated at the cathode by the electrolysis of the frozen water. The application of that concept relies on sufficient power diverted from a secondary battery (at −30 degrees Celsius, a voltage of 2.4 V must be applied to each fuel cell). Meanwhile, a secondary battery itself has very low performance characteristics at sub-freezing temperatures. A plot of the oxygen generation on the anode indicates that this design requires a more significant noble metal load in an anode catalyst than is used in modern fuel cells.
Japanese Patent No. JP-7169476 discloses a method of protecting a fuel cell against freezing of water by warming the fuel cell with an electrical heater, so that the temperature of a fuel cell does not fall below 0 degrees Celsius. However, if the fuel cell stops running for a long time, the amount of electrical energy required to protect the fuel cell becomes very significant. U.S. Patent Application Publication No. 2003/0162063 A1 teaches protecting a fuel cell against freezing by, first, draining water from the fuel cell to decrease its thermal capacity and, second, using an electrical or catalytic heater to keep a temperature of the fuel cell above 0 degrees Celsius. This approach also results in large amounts of electrical energy or fuel being required in the method.